A view showing a conventional structure of a PLL circuit having a circuit for evaluating the oscillation characteristics of a voltage controlled oscillator (hereinafter abbreviated as VCO) is shown in FIG. 11.
In FIG. 11, 1 is a phase comparator, 2 is a charge pump circuit, 3 is a loop filter, 4 is a voltage controlled oscillator (hereinafter abbreviated as VCO), 5 is a frequency division circuit, 7 is a test control signal, 64 and 65 are switches controlled by the test control signal 7 mentioned above, 6 is an input signal, 8 is a VCO input control terminal. The PLL circuit is composed of the phase comparator 1, the charge pump circuit 2, the loop filter 3, the VCO 4, and the frequency division circuit 5 which are connected in a loop-like configuration. The input 67 of the VCO 4 is connected to the VCO input control terminal 8 via the switch 64 controlled by the test control signal 7 and connected to the loop filter 3 via the switch 65 controlled by the test control signal 7. The VCO 4 mentioned above comprises a voltage-current conversion circuit 4a and a current controlled oscillator 4b. 
A description will be given herein below to an operation of the PLL circuit thus constructed.
In FIG. 11, in the case of operation as the PLL circuit, the test control signal 7 turns OFF the switch 64 and turns ON the switch 65 so that the input 67 of the VCO 4 is connected to the loop filter 3. As a result, the phase comparator 1, the charge pump 2, the loop filter 3, the VCO 4, and the frequency division circuit 5 are connected in the loop-like configuration to operate as the PLL circuit.
Next, in FIG. 11, in the case of evaluation of the oscillation characteristics of the VCO 4, the test control signal 7 turns ON the switch 64 and turns OFF the switch 65 so that the input 67 of the VCO 4 is connected to the VCO input control terminal 8. As a result, the VCO 4 is disconnected from the loop constituting the PLL circuit so that an output frequency is controlled by a voltage inputted from the VCO input control terminal 8. This allows evaluation of the oscillation characteristics of the VCO 4.
As a PLL circuit having such a conventional structure, there is known one disclosed in, e.g., Patent Document 1. In Patent Document 1, the PLL circuit is used to monitor the oscillation characteristics of a VCO with the structure described above and automatically adjust the oscillation characteristics of the VCO.
Patent Document 1: Japanese Laid-Open Patent Publication No. HEI 11-195983 (pages 9 to 11).